OSAKA -- Four people were found collapsed Thursday at a dry-ice store in Kita Ward here and were hospitalized, with two of them in critical condition and the other two seriously ill, police said.

Store owner Ken Iwamoto, 50, and his sister, Keiko, 55, are ill but able to speak, but his wife, Kikuko, 43, and another of his sisters, Kiyoko, 52, are unconscious and in critical condition, police said.

A part-time employee found them collapsed on top of one another, foaming at the mouth, in an office in the back of the store's work area at around 8:10 a.m., police said. They suspect the four suffered from a lack of oxygen, as the office was full of carbon dioxide at the time.

The door to the storage room where solidified carbon dioxide was kept was shut at the time they were found. A shutter in the store, which faces onto a street and is normally kept open, was closed, police said.

The store sells dry ice, which is cut from slabs of frozen carbon dioxide bought from a manufacturer, an employee of a company familiar with the business said.

According to the Osaka City Fire Department, people can suffocate from high-density carbon dioxide if dry ice vaporizes and fills a room that is poorly ventilated.